In a nutshell: Deficits in brain connectivity in people with corpus callosum dysgenesis may not be obvious when they are at rest or undertake simple tasks, but emerge only when they face complex tasks.

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The human brain is separated into two hemispheres that are connected by a structure called the corpus callosum. This thick band of nerve fibres transmits information back and forth between the hemispheres, which is necessary for many brain functions.

In some people, the corpus callosum does not develop properly. This condition, called corpus callosum dysgenesis (CCD), can have mild to severe effects on brain activity. The fact that CCD doesn’t completely prevent communication between the hemispheres suggests that the brain can create new, indirect pathways for transmitting information. However, researchers didn’t know whether the pathways shaped by CCD can support the same types of brain activity – and to the same extent – as the pathways in a normal brain.

To learn more, Brain Function CoE investigators Luke Hearne and Jason Mattingley from the Queensland Brain Institute, along with their colleagues, compared the brain activity of individuals with or without CCD while they completed tasks of varying difficulty.

Participants completed the Latin Square Task, which is a bit like a Sudoku puzzle. The cells of a 4×4 grid are populated with geometric shapes (square, circle, triangle and cross) or left empty, and one cell has a question mark. The participant must place a particular geometric shape in the cell with the question mark, making sure that each shape appears only once in each row and in each column. The complexity of the task varied depending on whether the participant needs to combine information from a single row or column, a single row plus a single column, or several rows and columns.

While the participants completed this task, the researchers measured their brain activity using magnetic resonance imaging. They looked at two different sets of brain regions and analysed the connectivity between them.

The researchers found that when the complexity of the Latin Square Task was low, both the participants with CCD and those with a normal corpus callosum had similar levels of brain activity and connectivity. However, when the complexity of the task increased, participants with CCD had less brain activity and connectivity compared with the other participants.

These findings explain why deficits in brain connectivity in people with CCD remain hidden when they are at rest or undertake simple tasks, but become obvious when they face more complex tasks. This also explains how CCD has varying effects on brain function despite the creation of new pathways in the brain.

Next steps:
The researchers plan to investigate the relationship between the new, indirect pathways formed in the brains of people with CCD and the mix of behaviours that result from this disorder.

Hearne, L.J., Dean, R.J., Robinson, G.A., Richards, L.J., Mattingley, J.B., & Cocchi, L. (2019). Increased cognitive complexity reveals abnormal brain network activity in individuals with corpus callosum dysgenesis. NeuroImage: Clinical, 21, 101595. doi: 10.1016/j.nicl.2018.11.005

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